A spectral classification system is a morphological device for arranging in order a finite series of observations of individual objects, One has thus three elements: an “order”, i.e. an abstract discontinuous scheme, a series of objects and a certain type of observation (spectrogram) that is performed on all objects of the sample. The “order” is in principle arbitrary; one could think for instance of an order based on richness of lines, or on the presence of hydrogen lines, etc. However such an idealization is clearly simplistic, because it would imply that the classifier is completely unaware of theory -but he is not, if for no other reason than simply that he would never get his PhD. Thus one should add the constraint that the scheme one adopts be physically sound.